Using a first IF of 43.5 MHZ or less in an FM radio in a television tuner

ABSTRACT

A television receiver includes a single tuner for tuning both television channels and broadcast FM stations. The tuner is operated at a fixed gain setting in FM reception mode. The tuner serves as the first conversion stage of a double conversion FM receiver, wherein the mixer of an FM radio integrated circuit serves as the second conversion stage. The arrangement operates at a specific and non-arbitrary first IF frequency. The FM receiver is also capable of automatically receiving National Weather Service broadcasts on that one of NWS&#39;s seven allocated frequencies which is operating in a listener&#39;s area. The receiver provides an on-screen display of the currently tuned FM channel number.

CROSS REFERENCE TO RELATED APPLICATIONS

Patent applications bearing attorney docket numbers: RCA 85,975, RCA85,976, RCA 86,099, RCA 85,961, RCA 86,109, and RCA 86,110, filedherewith, and assigned to the same assignee as the subject application,contain related subject matter.

FIELD OF THE INVENTION

The subject application concerns the field of television receiversincluding an FM radio.

BACKGROUND OF THE INVENTION

It is desirable to have a television receiver which is capable ofreceiving not only television signals, but also broadcast FM radiosignals. In the United States, the broadcast FM band occupies a band offrequencies extending from approximately 88 MHz to approximately 108MHz. This band of frequencies lies between the frequencies allocated forbroadcast television channel 6 and television cable channel 98. Modernintercarrier sound-type television receivers having the capability toreceive broadcast FM signals are known from the prior art. However, inthese known arrangements, their respective manufacturers added aseparate FM radio having its own tuner.

SUMMARY OF THE INVENTION

A single tuner in a television receiver is employed for tuningtelevision signals in at least one band of television frequencies, andbroadcast FM radio signals in an FM band of frequencies adjacent to thetelevision band of frequencies. The television tuner serves as the firstfrequency conversion stage of a double conversion FM receiver, whereinan FM radio integrated circuit serves as the second frequency conversionstage. It is herein recognized that in a double conversion FM receiver,the frequency of the first IF must be carefully chosen to avoid an imagefrequency with respect to the lowest frequencies of the FM broadcastband.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows, in block diagram form, a television receiver incorporatingthe subject invention.

FIG. 2A shows a parallel resonant FM trap as known from the prior art.

FIG. 2B is a graph of the amplitude vs. frequency characteristic of aparallel resonant circuit of the type shown in FIG. 2A.

FIG. 3A shows an FM trap in accordance with the subject invention.

FIG. 3B is a graph of the amplitude vs. frequency characteristic of theFM trap of FIG. 3A and antenna input circuitry, when the tuner is tunedto channel 6.

FIG. 4 is an illustration showing a display screen produced inaccordance with the invention.

FIG. 5 is an illustration of a portion of the tuner of FIG. 1, showingthe connection of the FM trap of FIG. 3A.

FIG. 6A shows the combined 43.3 MHz bandpass filter and 48.65 MHzhalf-IF trap of FIG. 1.

FIG. 6B is a graph of the gain vs. frequency characteristic of thefilter unit of FIG. 6A.

DETAILED DESCRIPTION OF THE EMBODIMENT

Referring to FIG. 1, television radio frequency (RF) and broadcast FMradio frequency signals are applied an to RF input terminal of an FMtrap circuit generally designated 100. FM trap 100 will be described indetail below with respect to FIG. 3. RF signals appearing at the outputof FM trap 100 are applied to a tuner 102. Tuner 102 includes an RFamplifier 102a for amplifying RF signals, and applying the amplified RFsignals to one input of a mixer 102b. Tuner 102 also includes a localoscillator 102c for generating a local oscillator signal which whenapplied to a second input of mixer 102b heterodynes with the amplifiedRF signal and produces an output signal at the television intermediatefrequency (IF frequency). Tuner 102 selects a particular RF signal undercontrol of a tuner control unit 104. Alternatively, tuner control unit104 may also be included within tuner 102. Tuner control unit 104applies a tuning control signal to tuner 102 via a wire 103, and appliesbandswitching signals via a control bus 103'. The tuning control signaland bandswitching signals control the frequency at which localoscillator 102c oscillates, thus determining which RF signal isconverted (heterodyned) to the IF frequency. Tuner control unit 104 iscontrolled by a controller 110. Controller 110, which may be amicroprocessor or microcomputer, includes a central processing unit(CPU) 112, a read-only memory (ROM) 114, and a random access memory 116.Controller 110 receives user-entered control signals from a localkeyboard 122, and from an infrared (IR) receiver 120. IR receiver 120receives and decodes remote control signals transmitted by a remotecontrol unit 125.

The intermediate frequency (IF) signal produced by tuner 102 is appliedto a surface acoustic wave (SAW) filter preamplifier 105 which amplifiesthe IF signal and applies it, via SAW filter 106 to a video signalprocessing unit 130. Video signal processing unit 130 comprises a videoIF (VIF) amplifying stage, an automatic gain control circuit (AGC), anautomatic fine tuning circuit (AFT), a video detector, and a second IF(SIF) amplifying stage. Processing unit 130 produces a basebandcomposite video signal (TV), and a sound carrier signal. The soundcarrier signal is applied to an audio signal processor unit 135 whichincludes a TV stereo decoder, a matrix, and a DBX expander. Audio signalprocessor unit 135 produces left and right audio signals and appliesthem to one pair of inputs of an audio switch unit 136. The output ofaudio switch unit 136 is coupled to an audio amplifier unit 137. Audioamplifier unit 137 produces amplified baseband left and right audiosignals and applies them to a pair of speakers 138 for soundreproduction.

The baseband video signal (TV) is coupled to a video processor unit 155and a kine driver amplifier 156, and ultimately displayed on a displayscreen of a display device 158. Video signals are also applied to a syncseparator unit 160 which derives vertical and horizontal synchronizingsignals therefrom. The derived vertical and horizontal signals areapplied to a deflection unit 170 for the production of deflectionsignals for application to the yoke assembly of display device 158.Under control of controller 110, an on-screen display processor 140generates character signals, and applies them to a second input of videosignal processor 155, for display on display device 158. The circuitrydescribed thus far, with the exception of the particular FM trap shownin FIG. 1, is known from the RCA CTC 156 color television chassis.

The intermediate frequency (IF) signal produced by tuner 102 is alsoapplied, via a 43.3 Mhz bandpass filter and 48.65 MHz trap arrangement145, to a single chip FM radio integrated circuit (IC)180. FM radio IC180 is, for example, a CXA12338M/S AM/FM Stereo Radio Circuitmanufactured by SONY Corporation. FM radio IC 180 includes an amplifier180a, a mixer 180b, an oscillator 180c, a voltage controlled oscillator(VCO) 180d, an FM IF and detector unit 180e, and an FM stereo decoderunit 180f.

It is herein recognized that television tuner 102 may be used as thefirst frequency conversion stage of a double conversion tuner for the FMbroadcast band, wherein the second frequency conversion stage of thedouble conversion tuner is provided by FM radio IC 180. That is, aparticular FM radio signal is selected and converted in frequency fromone of the FM radio band of frequencies, to a first intermediatefrequency of 43.3 MHz. The value 43.3 MHz is important and its selectionwill be discussed below.

The signals at the first IF frequency are then heterodyned in mixer 180bwith the 54.0 MHz oscillator signals produced by fixed frequencycrystal-controlled oscillator 180c. It was found that it is desirable tocrystal-control oscillator 180c to avoid drifts in frequency due totemperature changes which may occur in and around the area of oscillator180c. While a 54.0 Mhz crystal can be used, it was found that the thirdovertone (at 54 MHz) of a standard value 18 MHz crystal could be used aswell. The result of the heterodyning process is an FM radio signal atthe nominal FM IF frequency of 10.7 MHz, which is then filtered in aceramic resonator arrangement, generally designated 182. The secondceramic resonator of ceramic resonator arrangement 182 was added toimprove selectivity. Signals at the output of ceramic resonatorarrangement 182 are then amplified, detected, and decoded by FM signalprocessing units 180d, 180e, and 180f, in the normal manner. Apotentiometer VR1 is provided for adjustment of the VCO frequency.Decoded left (L) and right (R) stereo signals are applied to a secondpair of input terminals of audio switch 136. When the decoded left (L)and right (R) stereo signals are selected by audio switch unit 136, theyare applied to audio amplifier 137 for reproduction in speakerarrangement 138. Lines 117 and 118 coupled between FM radio IC 180 andcontroller 110 convey signals indicative of whether a signal is tuned,and whether a signal is in stereo, respectively.

Tuner 102 is of the frequency synthesis (FS) type, which means that thefrequency of the local oscillator can be changed in a series of steps ofa given size under control of controller 110. In FM reception mode,controller 110 causes oscillator 102c to change its frequency in 31.5kilohertz steps. This means that there can be a mistuning of an FMstation by a maximum of 31.5 kHz/2, or 15.75 kHz error. This isacceptable because FM radio IC 180 has acceptable demodulationcharacteristics over a range of approximately ±/- 110 kHz, and alsobecause the FM broadcast frequencies are spaced 200 kHz apart.

The selection of 43.3 MHz as the frequency for the first IF of thedouble conversion FM radio receiver will now be explained. As is wellknown, the amplitude vs. frequency characteristic of the tuner issubstantially shaped like a haystack, with the chroma carrier and pixcarrier residing at respective sides of the haystack approximately 3 dbdown from the maximum. The approximate center point of the haystackbetween these two carriers is 44 Mhz. One skilled in the art mightbelieve that this would be the optimum frequency for the first IF of theFM radio system. However, 44 Mhz is almost exactly the half frequencyvalue of the lowest FM radio frequency (at 88.1 MHz), and would causethe following problem. The frequencies of the signals applied to a mixerare doubled by the action of mixing. Most of these products are out ofband and filtered out by tuned circuits coupled to the output of themixers. If 44 MHz is used as the first IF frequency, then localoscillator 102c would oscillate at 132.1 MHz in order to tune an 88.1MHz FM carrier. In that case the following signals would be produced,

132.1 MHz-88.1 MHz=44 MHz (the desired signal); and

2×88.1 MHz-132.1 MHz=44.1 MHz (undesired image).

The undesired image signal is well within the bandwidth of the secondIF. This situation causes interference and distortion at the systemaudio outputs. This is further complicated by the fact that tuner FMtraps give very little attenuation at 88.1 MHz causing intermodulationdistortion in the tuner to happen at relatively low input signal levels.Frequencies greater than 44 Mhz, but less than the pix carrier at 45.75MHz, would cause image problems on higher FM radio stations. The bestvalue therefore is one below 44 MHz, but higher than the colorsubcarrier at 42.17 MHz (because going lower than the color subcarrierwould cause the signal to drop rapidly down the "haystack"). The value43.3 MHz is close enough to the crest of the haystack to providesymmetrical signals, and far enough away from 44 MHz to avoid imageinterference problems. When 43.3 MHz is selected at the first IFfrequency, local oscillator 102c would be controlled to oscillate at131.4 MHz in order to select an FM carrier at 88.1 MHz. This producesthe following output signals,

131.4 MHz-88.1 MHz=43.3 MHz (the desired signal); and

2×88.1 MHz-131.4 MHz=45.8 MHz (undesired image).

The undesired image signal is now 2.5 MHz away from the desired signal,is well outside the 300 kHz bandwidth of the second IF stage, and willnot cause distortion. In fact, a signal having a frequency between 43.5MHz and the color subcarrier frequency, is a good candidate for thefirst IF of the above-described double conversion tuner.

Similarly, the second IF has an image problem to be avoided.Specifically, a signal at 48.65 MHz (i.e., 43.3 MHz +5.35 MHz (one-halfthe second IF frequency of 10.7 MHz) would cause an image to appear at10.7 MHz, again causing interference. Because the second IF frequency isfixed at 10.7 MHz, this problem is eliminated in filter unit 145 withouthaving to employ tracking filters. The circuitry of filter unit 145 isshown in detail in FIG. 6A. The 43.3 MHz bandpass filter comprises api-type arrangement of an inductor L601, and capacitors C601 and C602. Atrap at 48.65 Mhz was obtained by adding a capacitor C603 in parallelwith inductor L601. The gain vs. frequency characteristic for thisarrangement is shown in FIG. 6B. The following component values arepreferred:

L601: 101 nanohenries

C601: 39 picofarads

C602: 120 picofarads

C603: 100 picofarads

In operation, controller 110 receives a command, via local keyboard 122,or via IR receiver 120, to enter the FM radio mode. In response,controller 110 applies a signal to the base of transistor Q1 viaresistor R1. Transistor Q1 switches on and provides a source of supplyvoltage to a voltage regulator circuit R2, D2 which in turn providespower (VCC) to operate FM radio IC 180. This switched VCC is alsoapplied to the control terminal of stereo switch 136 and causes theselection the FM radio audio signals in FM radio mode.

There are two obstacles to good FM reception performance, poorsensitivity and overload, and a carefully chosen compromise between thetwo must be utilized. Recall that in the television mode of operation,the RF-amplifier is gain controlled by an AGC signal derived in thetelevision video IF (VIF) circuitry. In FM mode, the AGC signals aredisconnected from the RF amplifier because no meaningful AGC signals arebeing produced in the VIF circuitry. If the television tuner were to beoperated at maximum gain in FM reception mode, medium to strong level FMsignals would overdrive the tuner mixer and RF stages, creating unwanteddistortion products. Providing a separate FM AGC arrangement is simplyunacceptable due to the cost and complexity which would be added to thetelevision receiver. The solution is to operate the RF stage of thetuner at a fixed gain during FM reception mode. This arrangement has amuch lower cost, adding only a few components. The gain reduction mustbe chosen carefully. Too much gain reduction would produce poor FMreception sensitivity, and too little gain reduction yields an overloadsituation. A second factor which aids to make operation of the RF stageat a reduced gain function well, is the fact that the noise FIGURE ofthe gain reduced RF amplifier stage is degraded (becomes higher) at amuch slower rate than the rate for gain reduction, thus maintaining abetter signal to noise ratio. This permits compensation for the RFamplifier gain reduction to be placed in a subsequent IF postamplifierstage, to maintain overall receiver sensitivity.

Disconnecting of the AGC signals is accomplished by applying the 4.7volt FM radio switched VCC to AGC line 102d via a diode D1. The FM radioVCC supply is well regulated enough to yield gain reductions which fallwithin acceptable tolerances. It is important to note that the FM radioIC chosen has a wide range of usable operating voltages. The 4.7 voltlevel was specifically chosen to fit the needs of the television tunerRF gain reduction bias. A resistor R3 isolates the AGC circuitry fromthe applied VCC. The amplitude of the switched VCC after passing throughdiode D1 is approximately 4 volts. Applying a fixed 4 volt signal to theAGC control terminal of RF amplifier 102a causes it to operate in alower gain mode.

The switched FM radio VCC is also applied to the base of SAW filterpreamplifier 105 to disable the amplifier and further attenuate unwantedsignals at the input of video processing unit 130.

Surprisingly, it was found that an FM trap produces a beneficial effectin a television receiver utilizing a single tuner for receiving bothtelevision signals and broadcast FM radio signals. Specifically, the FMtrap attenuates the FM radio signals which would otherwise have toogreat an amplitude at the television tuner input. It is also recognizedherein that the FM trap should exhibit a frequency response havingrelatively sharp "skirts" to minimize interference with signals ofadjacent television channels, and having a substantially flat bandreject region to provide FM signals having a substantially uniformamplitude throughout the FM radio broadcast band.

FIG. 2A shows a parallel resonant FM trap known from the prior art.Series resonant FM traps, and combinations of series and parallel FMtraps were also known in the prior art. In each case, however, no effortwas made to limit the attenuation of these prior FM traps. Instead, eachattempted to obtain the deepest possible notch, because in a televisionreceiver without an FM radio, there is no need to preserve any of thebroadcast FM signal spectrum. FIG. 2B shows the amplitude vs. frequencycharacteristic of a parallel resonant circuit, such as shown in FIG. 2A.This arrangement is unsatisfactory for a combined television and FMsystem for the following reasons. If the resonant frequency of thecircuit of FIG. 2A were to be set at the center of the FM band offrequencies, then the amplitudes of signals of the individual respectiveFM broadcast stations would vary greatly at the input of the RFamplifier. It is also unsatisfactory because the roll-off of thecharacteristic (i.e., the slope of the skirts) is not steep enough toprovide enough protection from FM interference for the adjacenttelevision channels.

Turning now to FIG. 3A, a three-section FM trap is shown which overcomesthe above-noted problems of the prior art FM traps. Section I of thethree-section FM trap comprises a parallel arrangement of an inductorL301, a resistor R301, and a capacitor C301. Section I is tuned to 97.5MHz to make the frequency response of the overall arrangement as uniformas possible. Section II of the three-section FM trap comprises aparallel arrangement of an inductor L302 and capacitor C302. Section IIis tuned to 104.0 MHz to provide protection for VHF channels 12 & 13 (inthe U.S.). Section III of the three-section FM trap comprises a seriesresonant circuit disposed from a point between Sections I and II, to apoint of reference potential (i.e., signal ground). Section III is tunedto 90.5 MHz to protect low band VHF channel 6 against educational FMtransmissions which are as close as 88.1 MHz. Resistors R301 and R303set the trap depth. It should be noted that Section II needs noadditional loading since the loading effects of the antenna filtercircuitry which follows it, reduces the trap depth of Section II to theamount desired. The above-described arrangement leaves the channel 6chroma carrier essentially unmodified in level, but pulls down thechannel 6 sound carrier about 3-4 db, which is felt to be acceptable.The cable channel A-2 (i.e., 98) picture (pix) carrier is reduced byapproximately 1 db, but this too is felt to be acceptable.

The following component values are preferred.

    ______________________________________                                        Section I  L301    approx. 18.3 nanohenries (adjust.)                                    R301    270 ohms                                                              C301    150 picofarads                                             Section II L302    approx. 16.2 nanohenries (adjust.)                                    C302    150 picofarads                                             Section III                                                                              L303    approx. 680 microhenries (adjust.)                                    R303    6.8 ohms                                                              C303    4.7 picofarads                                             ______________________________________                                    

The above-described three-section FM trap provides a uniform level ofrejection to signals in the 88 MHz to 108 MHz range of approximately10±4 db. during the FM reception mode of operation. When the tuner istuned to channel 6, however, the FM band rejections, as shown in FIG.3B, are in the range of 18-22 db due to the added selectivity of theantenna input circuitry. The response characteristic shown in FIG. 3Bwas measured at the drain terminal of the RF amplifier dual-gate FETtransistor Q501 of FIG. 5.

The three-section FM trap described above exhibits the following typicalperformance (referenced against the broadcast television channel pixcarrier).

    ______________________________________                                        Frequency (MHz)   Response (db)                                               ______________________________________                                        83.25   (chan 6 pix ref.)                                                                           -0                                                      86.83   (chan 6 chroma)                                                                             -0.2                                                    87.75   (chan 6 sound)                                                                              -2.7                                                    88.1    (lowest FM station)                                                                         -4.6                                                    88.3                  -5.4                                                    88.5                  -6.2                                                    88.7                  -7.1                                                    88.9                  -8.0                                                    89.1                  -9.0                                                    90.1                  -12.6                                                   90.5                  -12.2                                                   91.1                  -10.1                                                   92.1                  -8.2                                                    93.1                  -7.6                                                    94.1                  -8.0                                                    95.1                  -9.0                                                    96.1                  -10.3                                                   97.1                  -11.1                                                   98.1                  -10.8                                                   99.1                  -9.9                                                    100.1                 -9.0                                                    101.1                 -8.7                                                    102.1                 -9.2                                                    103.1                 -10.7                                                   104.1                 -11.8                                                   105.1                 -7.8                                                    106.1                 -3.3                                                    107.1                 -0.8                                                    107.9   (top FM station)                                                                            -0.4                                                    ______________________________________                                    

The desired end result of the trap attenuations by themselves, is that arequired reduction of overall tuner gain at the FM band of frequenciesis achieved. Compared to the average power gain of adjacent televisionchannels 6 and 98, the reduction of the overall tuner gain at thefollowing FM band frequencies is realized.

    ______________________________________                                                   TYPICAL AVERAGE LOSS RELATIVE                                                 TO THE AVERAGE POWER GAIN OF                                       FREQUENCY  ADJACENT TELEVISION CHANNELS                                       (MHz)      6 AND 98 (db)                                                      ______________________________________                                        88.1         7.5                                                              90.5       13                                                                 93.5       12                                                                 97.5       12                                                                 100.7        10.5                                                             104.1        6.5                                                              107.9       0                                                                 ______________________________________                                    

During FM reception mode no television images are available for viewing.Accordingly, when an FM station is selected controller 110 causeson-screen display processor 140 to display a message indicating to auser that the television receiver is in FM mode, that a particular FMstation has been selected and whether or not the received FM signal isin stereo. Such a display is shown in FIG. 4, wherein a message isdisplayed on the screen 410 of a television receiver 400. The display ispresented to the user for a predetermined period of time (perhaps 30seconds), after which there is displayed a blank screen.

FIG. 5 shows in detail the connection of the FM trap circuit of FIG. 3Ato a simplified version of a portion of tuner 102. Specifically, theoutput of FM trap 500 is coupled through a series inductor, to a seriesof traps, generally designated 510. Traps 510 have a first parallel traptuned to the TV IF frequency, a second shunt series trap to remove allsignals below channel 2, and a second parallel trap also tuned to the TVIF frequency. The output of traps 510 is applied to the input of RFamplifier, generally designated 520. A tuner of this type is known fromthe MTP-M2016 tuner used with the CTC-156 chassis manufactured byThomson Consumer Electronics, Inc., Indianapolis, Ind., and does notneed to be described in detail.

It is also recognized herein that the subject apparatus can e used totune broadcasts of the National Weather Service (NWS). These broadcastsare allocated to the following seven frequencies: 162.400 MHz, 162.425MHz, 162.450 MHz, 162.475 MHz, 162.500 MHz, 162.525 MHz, and 162.550MHz. Only one of these frequencies is assigned to a given geographicarea. Typical weather radio receivers provide a switch for selecting oneof three crystal controlled frequencies for receiving the broadcast.

It is further recognized herein that when NWS mode is selected only thecenter of the NWS band need be tuned. This is true for three reasons.First, the IF filters have a 190 kHz 3 db bandwidth which is greaterthan the 150 kHz total channel spacing of the NWS band. Second, thediscriminator used performs well, even ±1,000 kHz from the tunedfrequency. Third, there is little or no overlap of the NWS stations(because the NWS frequencies are reserved, and because there is only onetransmitter operating in a given area).

It is herein specifically recognized that the subject invention is alsouseful in videocassette recorders (VCRs). The term television receiver,as used herein, includes television receivers having a display device(commonly known as television sets) and television receivers without adisplay device, such as VCRs.

What is claimed is:
 1. A television receiver, comprising:tuner means foroperating in a first mode for receiving television RF signals, saidtuner means selecting a particular television RF signal from a pluralityof television RF signals in response to a control signal; control meansfor generating said control signal for causing said tuner means toselect said particular RF signal; said tuner means also operating in asecond mode as a first frequency conversion stage for a doubleconversion broadcast FM radio signal receiver, for receiving broadcastFM radio RF signals, said tuner means selecting a particular broadcastFM radio RF signal from a plurality of broadcast FM radio RF signals inresponse to said control signal for converting said broadcast FM radioRF signal to a first intermediate frequency; a second frequencyconversion stage of said double conversion broadcast FM radio signalreceiver, said second frequency conversion stage receiving saidbroadcast FM radio signal at said first intermediate frequency andconverting said broadcast FM radio signal to a second intermediatefrequency; and means for demodulating audio signals from said broadcastFM radio signal at said second intermediate frequency; and wherein saidfirst intermediate frequency is 43.3 MHz.
 2. A television receiver,comprising:tuner means for operating in a first mode for receivingtelevision RF signals, said tuner means selecting a particulartelevision RF signal from a plurality of television RF signals inresponse to a control signal; control means for generating said controlsignal for causing said tuner means to select said particular RF signal;said tuner means also operating in a second mode as a first frequencyconversion stage for a double conversion broadcast FM radio signalreceiver, for receiving broadcast FM radio RF signals, said tuner meansselecting a particular broadcast FM radio RF signal from a plurality ofbroadcast FM radio RF signals in response to said control signal, forconverting said broadcast FM radio RF signal to a first intermediatefrequency; a second frequency conversion stage of said double conversionbroadcast FM radio signal receiver, said second frequency conversionstage receiving said broadcast FM radio signal at said firstintermediate frequency and converting said broadcast FM radio signal toa second intermediate frequency; and means for demodulating audiosignals from said broadcast FM radio signal at said second intermediatefrequency; and wherein said first intermediate frequency is less than orequal to 43.5 MHz and greater than 42.17 MHz.